1. Field of Invention
This invention relates to adhesives. More particularly, this invention relates to two-part room-temperature curing methacrylate based adhesives that are used to bond a wide variety of materials, including thermoset plastics, thermoplastics, metals, wood, ceramics and other materials and combinations of materials. This invention involves a significant improvement in the ability of adhesives to bond certain difficult-to-bond composite materials with minimum required surface preparation. Another feature of this invention is the high degree of elastic behavior of the cured adhesives and the ability of the cured adhesive materials to retain a high proportion of their elastic behavior after exposure to elevated temperatures or long-term aging.
2. Background Art
The three common classes of two-part room temperature curing reactive adhesives are epoxies, polyurethanes, and acrylics. The discussion of these prior art adhesives and the inventive adhesives emphasizes structural applications, wherein a very strong bond is achieved between two structural members of an assembly, and the bond is often strong enough to cause failure of the material before the assembly fails. However, all of these adhesive materials, can be and are used to advantage in less demanding applications, as well, in which case one or more of the advantages of the particular adhesive fulfills a specific bonding requirement.
Epoxy adhesives, which are the earliest, best known and among the most common structural adhesives in general use, consist of an epoxy resin adhesive component and an amine, polyamide, or combined amine and polyamide hardener components. Faster curing epoxies can be formulated with polymercaptan hardeners that are generally used in combination with polyamide and amine hardeners.
Polyurethane adhesives generally consist of an isocyanate-terminated polyol and a hardener or curative component that consists of a polyol or amine or a combination of polyols and amines.
The epoxy and polyurethane adhesives cure upon mixing when the hardener component reacts with the epoxy or polyurethane resin component in an addition polymerization process.
Methacrylate or acrylic adhesives that are used in the same applications as epoxies and polyurethanes generally consist of a polymer-in-monomer solution of an elastomer or thermoplastic resin or a combination thereof in a monomer such as methyl methacrylate. Hardening is achieved when a combination of a peroxide and an amine is introduced into the polymer-in-monomer mixture to initiate a free-radical curing reaction. Generally, the adhesive component contains either the amine or peroxide component and the co-reactive peroxide or amine component is mixed with the adhesive just prior to bonding.
Each of the three common reactive adhesive classes has characteristic advantages and disadvantages. For example, epoxies tend to be characterized as safe and relatively easy to mix and apply, but tend to be somewhat rigid and sensitive to cleanliness of the surface to be bonded. Polyurethanes are generally considered to be much more flexible and elastic, but also suffer from sensitivity to surface contamination, moisture and humidity. Both of these adhesive types have the limitation that fast-curing products tend to have very short open working time after mixing, and products with more acceptable open time have very long cure times. This limitation is imposed by the linear reaction mechanism that is characteristic of the addition polymerization reaction by which they cure.
In terms of the characteristics of the cured adhesive and resulting bond, epoxies are considered to be very strong because of their high modulus or rigidity and resulting high lap shear strength. They are generally recommended for bonding metals because of their affinity for metal surfaces and high shear strengths. However, their rigid nature limits their usefulness in applications that require flexibility in the adhesive bond. Epoxies also have limited ability to bond thermoplastic materials.
Polyurethanes are generally much more elastic, tough and flexible than epoxies. Elasticity, toughness and flexibility are beneficial when adhesive bonds are subjected to peeling or impact forces, and when bonds and bonded assemblies are subjected to dynamic fatigue stresses. However, polyurethanes are not as useful as epoxies for bonding metals, and are generally more suitable for bonding plastic materials in applications that are subjected to bending and impact stresses.
Two-part acrylic or methacrylate adhesives overcome two of the major drawbacks of the epoxies and polyurethanes. They are much more tolerant of unclean or unprepared surfaces, and they have a much more favorable cure profile in terms of open working time and cure rate. In addition, they exhibit equal or better affinity for metal and plastic surfaces than either epoxies or polyurethanes. However, some materials, in particular certain composite materials, are difficult to bond in the “as received” condition. Specific examples include certain gel coats, which are highly crosslinked and inert polyester compounds that form the outer or “show” surface of fiberglass reinforced polyester (FRP) composite materials used to fabricate boats and other structures exposed to outdoor weathering.
Other examples are closed molded polyester composites, which are materials formed by processes other than the open molded processes used to produce FRP composite structures. Examples of closed molding processes and materials are sheet molding compounds (SMC), resin transfer molded (RTM) composites and pultruded composites.
The essential issues with closed molded processes and products are (1) these processes produce polyester composite articles with reduced emission of and worker exposure to the styrene component in polyester resins and are rapidly replacing open molded processes, and (2) these materials are generally characterized by resistance to the solvating effect of the methacrylate monomers that normally soften or penetrate the bonding surface prior to hardening of the adhesive. In addition, many of these materials use processing aids to provide smooth surfaces for painting. These materials can also interfere with the bonding process.
Other materials are used to facilitate release from the molds used to fabricate parts from them. Such materials are often added directly to the molding compound, in which case they are referred to as “internal” mold releases. Other materials may be sprayed on to the mold surface prior to molding. These materials are referred to as “external” mold releases. All of these processing aids can interfere with the formation of strong adhesive bonds.
The problems experienced in bonding these materials with prior art methacrylate-based adhesives, as well as the additional and undesirable processing steps required to use them, including grit-blasting, sanding, solvent wiping and priming are described in detail in U.S. Pat. No. 3,838,093, which is discussed in further detail below.
Epoxy adhesives based on standard DGEBA (diglycidyl ether of bisphenol-A) resins, cured with hardeners based on combinations of amines, polyamides and other additives used to impart specific properties, have effectively been used to bond some closed molded composite materials. However these adhesives do not completely cure at room temperature, and generally require thermal post-curing to develop their full physical strength.
Recent developments in polyurethane adhesive technology have been directed toward improving adhesion to these composite materials as disclosed, for example, in U.S. Pat. Nos. 5,340,901 and 5,548,056. However, as with epoxy adhesives, these materials often require thermal post curing. Even though polyurethanes do ultimately develop their full physical strength at ambient temperatures, such post curing may be required to meet process speed requirements or to develop full, reliable or reproducible adhesion to the composite surface, or both. In some cases, solvent-based primers are used to develop adhesion at ambient temperatures, but this is undesirable for environmental and health reasons.
Yet another problem with the epoxy and polyurethane-based adhesives is their limited ability to bond to open-molded composite surfaces. Open-molded composite articles are fabricated by using a combination of spraying and rolling processes that combine woven and chopped fiberglass with polyester laminating resins in open molds. A mold in the shape of the article is first sprayed with a gel or outer coat of a highly crosslinked, pigmented resin that creates a smooth, shiny exterior or “show” surface. The laminating resin and glass are then applied together in successive applications until the desired part thickness is achieved. It is desirable to be able to de-mold the molded article as quickly as possible and move it on to the assembly area. At this point, and for several hours thereafter, the resin is not completely cured and is referred to as “green” as the final stage of the polymerization process proceeds. In this state, the exposed or “raw” resin surface is difficult or impossible to bond with conventional epoxy or polyurethane adhesives.
Thus, while epoxies and polyurethanes are sometimes capable of bonding the gel coat or show surfaces of these resin structures, in most cases it is necessary to bond the raw surface to itself or to the gel coat surface. Methacrylate adhesives have been increasing in popularity and usage because of their ability to bond the raw fiberglass surfaces, even in the green state. However, as improvements and changes have been made in the composition of the gel coat materials in recent years, the ability of the methacrylates to bond them, especially in the “as received” condition, has become less predictable.
A significant characteristic of some of the more recent acrylics or methacrylates is elasticity, toughness and flexibility that is greater than that of epoxies and even approaches that of the polyurethanes. However, not all of the methacrylate adhesives exhibit such elasticity, toughness and flexibility initially, and many of those that do often fail to retain these properties over a long period of time or when heated to elevated temperatures. Such reduction in elasticity can be demonstrated by comparing the bulk stress-strain properties of films prepared from the compositions which have not been exposed to elevated temperatures with similar films that have been subjected to brief or prolonged exposure at various temperatures. Loss of elasticity that occurs upon brief exposure at elevated temperatures may be the result of a continuation of the curing process, or a “post curing” process. It is also believed that certain physical changes in the phase distribution of the glassy and rubbery components or domains can occur in the cured composition when it is heated to or above its glass transition temperature or Tg. Loss of elasticity that occurs upon prolonged exposure to elevated temperature can also be the result of either the post curing or physical processes described above or the chemical degradation because of oxidative or other thermally induced reactions that adversely affect the polymer structure.
The improvements of this invention are primarily directed toward changes that occur as a result of post curing or physical changes after relatively brief exposures to elevated temperatures or longer exposures to ambient temperatures. Resistance to oxidative or other thermal degradation processes is subject to other chemical formulating principles well known to those skilled in the art.
Practical manifestations of these phenomena include the potential loss of the ability of adhesive bonds to resist peeling or impact forces as the bonded assembly ages, or a reduction in the elasticity or increase in hardness of the cured composition in the center portion of a thick cross section. The latter is believed to occur when the exothermal heat generated by the polymerizing composition raises the temperature to a level that approaches or exceeds the Tg of the cured composition. Whatever the cause of the physical changes that occur during or after the initial curing phase of the adhesive, the ultimate physical and elastic characteristics of the adhesive can generally be predicted by a brief exposure of the material in question at an elevated temperature. Typical thermal exposures for this purpose are from 30 minutes to a few hours at temperatures ranging from about 70 degrees centigrade to about 100 degrees centigrade.
As the use of adhesives increases in the fabrication of composite structures, design engineers are increasingly aware of the need to reliably predict the physical characteristics of the adhesives, which become an integral part of the structure. In this regard, adhesives are generally characterized by their tensile strength, modulus or stiffness, and elastic properties. In some cases, a stiffer adhesive is desired in order to provide a high degree of load bearing capability in the bonded joint. In other cases, a more flexible or elastic joint may be required in order to resist shock, vibration and fatigue loads. In civil engineering applications, such as highway bridge construction, a somewhat stiffer bond may be desirable. However, it is important that the adhesive also have a predictable degree of elastic behavior in order to withstand the bond stresses that occur as a result of thermal cycling and the resulting differential expansion of the bonded components. It is also necessary to withstand the cyclic loading of the structure imposed by vehicular traffic on the bridge. In extreme cases, seismic loads on civil engineering structures demand the utmost in stress to failure capability of the adhesive and the resulting joint.
In the fabrication of boats, on the other hand, more flexible adhesives are often desirable. An example is the bonding of stringers or liners in the structural fabrication of the boat hull. In this application, there can be a combination of severe shear, peel, and shock loading of the bonds when the boat is operated at high speeds over rough or choppy water. Flexible adhesives can provide very durable joints by resisting the peel and shock loads imposed on the bond and by damping the energy transferred to the joint as it undergoes cyclic loading.
In all of these cases, it is imperative that the adhesive be capable of retaining the physical characteristics, especially the ability to withstand shock and fatigue loads, during the lifetime of the structure. It is further important that the components of the adhesive can be adjusted to provide the desired degree of stiffness and flexibility for a variety of applications.
It is clear from the discussion above that there is a need for adhesives that will reliably and predictably bond a wide variety of composite surfaces in the as received condition, rapidly and without the application of heat to complete the cure or develop full adhesive bond strength. It is also desirable for such adhesives to bond other structural materials such as metal, thermoplastics, wood, etc. It is further desirable that such adhesives possess a high and predictable degree of elasticity and retain their elasticity when exposed to elevated temperatures during the curing process or in service.
The benefits derived from the improvements of this invention apply to structural adhesive bonding applications. However, the compositions disclosed herein may also be useful in a number of other applications for which epoxy, polyurethane, methacrylate and polyester resins are employed. One such application is coatings. A specific example of a coating is the driving surface of bridge decks, including FRP or composite bridge decks which may be fabricated and assembled using adhesives, including the adhesive compositions of the present invention. Such coatings are often referred to as bridge deck overlays.
FRP or composite bridges and bridge decks have been developed to replace traditional steel and concrete structures for a number of reasons, including their resistance to rust and decay in severe climates. Their light weight and high strength relative to steel and concrete structures make them especially useful for reconstructing aged, deteriorated bridge structures. Composite decks can be used to replace the existing concrete and steel deck of a deteriorated structure without replacing the entire structure. The light weight and equivalent or superior load-bearing capability of the composite deck allows the bridge to support the same traffic loads as the existing structure without replacing other deteriorated structural support members of the bridge.
Reconstructed bridges, as well as new and replacement bridges can be installed more quickly and with less traffic disruption than traditional bridges. Other benefits of this concept are too numerous to mention and beyond the scope of this invention. However, a common problem with these applications is the final step of replacing the pavement or driving surface of the bridge deck.
Existing bridge resurfacing materials are generally composed of polymer latex modified concrete or an aggregate composition that uses an epoxy resin or polyester resin as a binder for the aggregate and adhesive to secure the surface to the bridge deck. These materials were originally developed and have been used for resurfacing traditional concrete or asphalt bridge decks. Application of these materials to composite bridge decks has been less than satisfactory for a number of reasons, including mismatched coefficient of thermal expansion relative to the composite deck, insufficient toughness and flexibility, poor or marginal adhesion, and complexity in mixing and application.
The benefits of the present invention for adhesives, namely elasticity and toughness, and the retention of elasticity and toughness, combined with the ability to bond a number of surfaces, including difficult to bond composites, are useful for replacement of the driving surface of the bridge as well. Their toughness, flexibility and resistance to cracking also provide potential benefits for overlay coatings for restoration of existing concrete and asphalt bridge deck surfaces. In this case, the coating can perform both as a traffic wear surface and as a sealant to prevent intrusion of moisture, salt and other damaging elements that can damage the concrete and metal bridge structure beneath the pavement. For this application, it is imperative that the coating be resistant to cracking or any other loss of integrity that allows moisture or damaging agents such as deicing chemicals, oils or fuels to penetrate the coating. Such penetration can eventually lead to disbondment of the overlay and or damage to the structural components of the bridge deck and supporting structures.
U.S. Pat. No. 3,333,025 discloses improvements in the adhesive properties of polymerizable adhesives based on mixtures of methyl methacrylate monomer, styrene monomer, polychloroprene, and optionally an unsaturated polyester resin.
U.S. Pat. No. 3,838,093 describes problems associated with the bonding of fiberglass reinforced polyester (FRP) substrates with adhesives, including the adhesives of the '025 patent. It further discloses compositions of primers based on isocyanate and polyol components as primers, wherein the primers require volatile organic solvents in order to be effectively applied. It further discloses the requirement to cure the primer by allowing it to stand at ambient temperatures for up to 72 hours, or by baking the primed substrate in an oven at 200-280 degrees F.
U.S. Pat. No. 3,890,407 discloses methacrylate adhesives with improved adhesive properties comprising mixtures of chlorosulfonated polyethylene (CSPE) in methyl methacrylate (MMA) monomer. Among the compositions disclosed are mixtures of Hypalon 20 and Hypalon 30 CSPE in MMA with other additives to complete the adhesive formulations. Among the improvements cited are increased speed of cure, improved adhesion to unclean or unprepared surfaces, and high bond strength.
U.S. Pat. No. 4,126,504 discloses methacrylate monomer based adhesives containing a variety of polymers, including polychloroprene, chlorosulfonated polyethylene, and butadiene/acrylonitrile. It suggests that mixtures of such polymers may be employed, but does not cite or claim specific mixtures or combinations of polymers or suggest or disclose specific advantages obtainable through the use of such mixtures. In particular, it does not suggest mixtures of polychloroprene or chlorinated polyethylene polymers with butadiene-acrylonitrile polymers.
U.S. Pat. No. 5,206,288 discloses methacrylate adhesives based on mixtures of a number of elastomers blended individually with a core-shell impact modifier. These adhesives exhibit a high degree of toughness and flexibility, especially at low temperatures. Polychloroprene and butadiene-acrylonitrile elastomers are disclosed individually in combination with the core-shell impact modifiers, but there is no suggestion of employing blends of these elastomers in combination with the impact modifiers.
It has now been discovered that unique and highly beneficial adhesive characteristics can be achieved by blending chlorinated polymers such as polychloroprene, chlorinated polyethylene and chlorosulfonated polyethylene with butadiene-acrylonitrile elastomers and methacrylate monomers and free-radical catalysts to form polymerizable methacrylate adhesives. Such adhesives display excellent adhesion to difficult-to-bond composite surfaces, without the need for extensive surface preparation. Moreover, the adhesives exhibit a high degree of elasticity and retain their elasticity following exposure to heat.